It is a once-in-a-lifetime opportunity. Later this year, Tony Blair will stand down and the Labour party will elect a new leader. The lucky winner of that contest also gets to be Prime Minister. For a chance to win this fabulous prize, all a Labour MP has to do is stand in an election. So far, only three candidates have expressed an interest: Gordon Brown, the Chancellor, John McDonnell, a maverick left-wing backbencher, and Michael Meacher, a former minister, also from the left of the party.

Mr Meacher has long been expected to run, but he only formally declared his ambitions last week. We are glad to see Mr Meacher's hat in the ring, not because he would make a good Prime Minister or because he has a hope of winning - he hasn't - but because he has shown that he believes the job is worth having and he is not afraid of losing. It is hardly surprising that a career politician should aspire to the highest office in the land. But it is baffling that no cabinet minister past or present seems to share that aspiration. When Margaret Thatcher resigned, men of substance in the Tory party quickly leapt on the chance to succeed her. By contrast, the big beasts of the Labour party are locked in a footling battle for second prize - the deputy leadership, a party post with no constitutional powers in government. There are six declared candidates for the job, three of them in the cabinet.

There are various explanations for this modesty. First, no one apart from Gordon Brown fancies being Prime Minister. But it is implausible that the cabinet is full of people who scrabbled up the power ladder with the intention of always stopping a few rungs down from the top.

Second, everyone is convinced that the Chancellor is the best candidate and no one wants to deprive Britain of the benefits of being led by him. That, in essence, is what senior Labour MPs say when asked in public why they do not stand. It is not what they say in private.

Third, no one thinks they can beat Mr Brown. That may well be true, but it is a poor reason not to try. Any leadership candidate would have a unique chance to broadcast their message to the country, assuming, that is, they have something to say. A good performance would guarantee a consolation prize of some top cabinet job. Or at least it should do, if the winner is capable of magnanimity and tolerant of colleagues with high profiles.

That leads to the last interpretation of the cabinet's restraint: they do not think Gordon Brown capable of political generosity. They have judged that standing, even to come a close second, would be suicidal. They fear a contest so vicious that, instead of clearing the air it would poison the atmosphere, leaving voters appalled by the stench. But the condemnation of Mr Brown's leadership qualities implied by fear of running against him is also damaging. The public can distinguish between the sort of unity that is born of loyalty and that which comes from cowardice.

Paucity of debate means also that the deputy leadership election is shaping up to be a drab parade of stunted ambition, a conference sideshow of apparatchik introspection with nothing to add to the conversation about what direction the country, rather than just the Labour party, should take.

By contrast a proper battle for the leadership would put some wind into the sails of a becalmed government.

It would give the Chancellor the opportunity to refute his critics, demonstrating that he can respond nimbly to the quick-fire battle of an election campaign and be graceful in victory.

If he is not given the chance to show those qualities, voters will be entitled to assume the worst: that their new Prime Minister is a bully and that, after 10 years in power, the Labour party cannot muster from its ranks more than one heavyweight player with the guts and imagination required to even want to lead the country.